The membranes which are employed for these purposes usually comprise various organic polymers or mixtures of organic polymers either alone or supported on a porous backing material. For example, semipermeable membranes which are used in desalination processes can comprise cellulose acetate polymers, thin film composite membranes comprising polymeric compounds such as polyethyleneimine, epiamine, polyvinylamine films composited on a porous support such as a polysulfone membrane, etc. Likewise, gaseous separation membranes may comprise polymeric membranes of cellulose nitrate, cellulose acetate, polydimethylsilicone, polystyrene, and poly(4-methyl-1-pentene), as well as thin film membranes such as polydimethylsilicone, polystyrene, polymethylpentene polymers composited on a porous support such as polysulfone, polyphenylene oxide, etc.
Some prior patents have shown certain polymeric compositions which are useable in various processes. For example, U.S. Pat. No. 4,071,454 discloses a hollow fiber which is useful for dialysis. However, a dialysis membrane which comprises a plurality of poly(vinyl alcohol) fibers is substantially different from the polymer blends of the present invention in which the latter may be used for a gas-separation. The fiber membranes which have been disclosed in this patent cannot and will not separate a gas inasmuch as they are utilized to physically absorb a whole molecule, that is, the undissociated molecule in the polymer, and subsequently moving said whole molecule through the polymer under the influence of an osmotic pressure gradient. In this respect, the polymer membrane is very similar in nature to a desalination membrane. In like manner, U.S. Pat. No. 4,264,676 discloses superfine fibers of the poly(vinyl alcohol) type which are prepared by baking these superfine fibers in the presence of a dehydration catalyst which may comprise ammonium polyphosphate, phosphoric acid, ammonium phosphate salts and hydrogen chloride to yield a polyenized poly(vinyl alcohol) [PVA] containing polyene structural units and vinyl alcohol units of varying ratios. This polyenized PVA can be chemically modified with sulfuric acid to yield a strongly acidic cation-exchange possessing crosslinked sulfate, sulfonic acid and sulfuric ester radicals. Further, the polyenized PVA can be chemically modified via Diels-Alder's reaction with maleic anhydride or acrylic acid to yield a weakly acidic cation-exchange fiber. Alternatively, the polyenized PVA can be chemically modified with epichlorohydrine, followed by amination with trimethylamine to yield a strongly basic anion-exchange fiber.
A thin film polymeric membrane may be used in a gas-separation process. The polymeric material will transport an ion such as a proton through the membrane in a charge transport reaction. In order to ultimately transport a molecule of hydrogen from one side of this membrane to the other, two protons must be formed. The protons are formed by dissociation of molecular hydrogen on a suitable electrode, the protons migrate through the polymeric medium and the corresponding electrons through an external circuit. The transported protons and electrons are then recombined, on a suitable catalyst, on the membrane's opposing surface to form molecular hydrogen. The membranes and fibers which were taught in the two previously mentioned patents do not dissociate the molecule or salt, but merely provide a means of transporting the nondissociated molecules through the membrane or ion-exchanging the salts within the fiber, respectively.
The membrane of the present invention is formed from an interpenetrating polymer network which is stable to moisture and therefore may be used as a hydrogen sensor. Interpenetrating Polymer Networks (IPN) may be prepared from two polymers, that is, a host and a guest polymer. There are usually three general classes of IPN's:
1. Sequential PA1 2. Simultaneous PA1 3. Latex
All three of these IPN's are similar in nature but differ only in the method of preparing the product. In the sequential type of IPN's, the host polymer is prepared in the absence of the second monomer. Following the preparation of the host polymer, it is then treated with the second monomer in the presence of a compatible solvent. Following the removal of the solvent the second monomer is then polymerized to yield the resulting guest polymer and the resultant IPN. The new polymer network is more than just a blending of the host polymer and guest polymer; it is the generation of a new polymeric system which possesses properties which are a combination of the host and guest polymer. In a system of this type, the host and guest polymer show only a physical interaction between the two polymers, there being no chemical or covalent bonding of the two polymer chains. This physical interaction between the two polymers in the IPN means that the polymer chains become permanently entangled with one another and therefore, it is impossible to leach or dissolve one polymer away from the other polymer.
In a majority of cases which are found in prior operations, an admixture of an organic compound, especially in a polymeric state, with an inorganic compound, results in a phase separation due to the fact that the two systems are immiscible in nature. However, we have now discovered that an IPN may be fabricated by admixing an inorganic compound such as a phosphoric acid or sulfuric acid with an organic polymer to form host polymers and thereafter subjecting this host polymer to further admixture with a guest polymer formed from a monofunctional monomer and a difunctional cross-linking agent. It was thoroughly unexpected that a thin film membrane could be cast from the IPNs to provide a membrane which would be highly conductive to protons and therefore find a use in separations which involve the generation of a proton as in the case of hydrogen.